Introduction to Mechanisms

Reactivity

Did you know that not all molecules are reactive? Only certain types of molecules will want to react in a mechanism. Let’s dig a little deeper into this...

Stability and reactivity generally have an inverse relationship. If a molecule is unstable in some way, it will want to react! Here are the 4 signs we can look for that determine reactivity:

These reactive trends ARE in order of strength (i.e. a formal charge will typically be more reactive than a dipole).  

Example: Are any of the following 4 molecules reactive?  

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Again, we are literally just matching these molecules to the 4 patterns discussed above. 

Nucleophiles vs. Electrophiles

Now that we know how to determine if molecules are reactive, we still don’t know HOW they will react! There are two major subtypes of reactivity that we’ll often use in Orgo 1 and 2: 

Note that a molecule doesn’t require a negative charge to be a nucleophile, but it needs to have similar properties (i.e. a source of electrons).

That said, try to identify if the following three molecules are nucleophilic or electrophilic. 

Example: Are the following 3 reactive molecules nucleophiles or electrophiles?    

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So that wasn’t so hard, but those were the easy cases. What if you have nucleophilic AND electrophilic regions on the same molecule? Is it possible to determine how it will react? Yes it is!

Rule: The side of the dipole with the highest bonding preference (the atom that wants to make the most bonds) will determine how the molecule reacts. 

Example: Are the following 3 reactive molecules nucleophiles or electrophiles?    

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Bond Making

Now we understand which molecules will want to react, and we are getting better at determining If they are nucleophiles or electrophiles, but how to they actually attack other molecules?

Reactive molecules share electrons to become more stable. Arrows are used to show which direction they are going.

  • Arrows always move from regions of high electron density to low electron density
  • By that logic, nucleophiles must always attack electrophiles.
  • Each attacking arrow represents two electrons being shared.
    • After the reaction is complete, replace that arrow with a new σ -bond

Summary: Molecules with lots of electrons will attack (draw an arrow to) molecules with a positive charge. Let’s get drawing!

Example: Draw the first arrow of the following mechanism 

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Example: Draw the first arrow of the following mechanism 

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Bond Breaking

So now we know how to make bonds. Do we ever have to break bonds? How do we know if we do or we don’t?

Bond breaking is sometimes required in mechanisms, but only when it is necessary to preserve octets.

Out of these two different ways, we will stick to heterolytic cleavage for the foreseeable future (we won’t discuss radicals for a few more chapters). 

Example: Identify if the following reaction requires a bond to be broken. Draw the products.

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Example: Identify if the following reaction requires a bond to be broken. Draw the products. 

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